Automatic volume control circuits



July 27, 1937Q w. VAN B. ROBERTS 2,088,179

AUTOMATI'G VOLUME CONTROL CIRCUITS Filed Oct. 20, 1934 3 Sheets-Sheet l 'le g agg l 1 l wml l l f. f 1| f f l v +5 l +19 Fig 1 4 ATroRNEY July 27, 1937. w. VAN B. ROBERTS l 2,088,179

AUTOMATIC VOLUME CONTROL CIRCUITS Filed Oct. 20, 1934 5 Sheets-Sheet 2 aL/f1.4@ aff.

4 s ww lNvENToR ML75/6 MA 3./7015E ATTORNEY July 27, 1937. w. VAN B. ROBERTS AUTOMATIC VOLUME CONTROL `CIRCUITS Filed Oct. 20, 1954 Z5 Sheets-Sheet 3 z3 aff 24 Patented July 27, 1937 prurito stares italiani FFME Walter van B. Roberts, Princeton, N. li., assigner to Radio` Corporation of America, a corporation of Delaware Application October 20, 1934, Serial No. 749,204

1 Claim.

My present invention relates to automatic gain control circuits for radio receivers, and more particularly to improved and novel types of automatic volume control circuits adapted for use in connection with radio broadcast receivers.

In my U. S. Patent 1,913,959 of June 13, 1933, there have been shown various types of auto-- matic volume control circuits for radio broadcast receivers. In general the purpose of these various circuits is to automatically control the amount of amplication in a receiving circuit, the systems disclosed in the patent utilizing the strength of a carrier wave, rather than the maximum wave amplitude, whereby the radio frequency amplication is not varied by audio irequency variations of the incoming wave amplitude, which would reduce the eiectiveness of demodulation, but is varied only by variation of carrier wave amplitude which may be due to general fading eiiects, or to tuning in of diiierent stations. Furthermore, in the said patent there are disclosed different types of automatic volume control circuits utilizing a magnetron as the amplification control element, the direct current component of detected signal energy being employed to regulate the operation of the control magnetron.

One of the main objects of the presentiinvention is to provide automatic volume control circuits of the general magnetron type disclosed in my aforementioned patent, and wherein the magnetron control circuits are improved so as to maintain full amplification in the high irequency amplier stages of a radio receiver up tothe point where the input to the detector reaches a predetermined value after which the automatic volume control mechanism rapidly reduces the amplication until at a value slightly greater than the said predetermined value the amplification becomes substantially zero.

Another important object of this invention is to provide 'an automatic volume control circuit for a radio receiver, the control mechanism utilizing a space discharge device of the magnetron type, and wherein the voltage input to the detector is maintained constant witlnn narrow limits for all signals whose strength exceeds the value required in order that the amplifying system may bring them up to the desired level oi detector input.

Another object of the invention is to provide an automatic volume control network for a radio broadcast receiver7 the network utilizing a Vspace discharge device of the magnetron type, and the volume control system being constructed in such `duced to its mostV essential networks.

(ci. 25e-2c) a manner that it is not necessary to provide a higher rectified voltage than required for normal operation of the receiver.

Still another object of the invention is to provide an improved volume control circuit for a radio receiver wherein variations of current flow in the space current path of the signal detector are utilized to regulate the operation oi a space discharge device of the magnetron type, and wherein the magnetron is adapted to vary the operating potential of a positive grid electrode, such as the screen grid, of a signal amplier of the receiver.

Still other objects of the invention are to improve generally the simplicity and efficiency of automatic volume control circuits of the magnetron type for radio receivers, and particularly to provide such circuits which are not only reliable and eilicient in operation, but economically constructed and assembled in radio receivers.

The novel features which l believe to be characteristic oi my invention are set forth in particularity in the appended claim, the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with thedrawings in which l have indicated diagrammatically several circuit organizations whereby my invention may be carried into effect.

In the drawings:-

Fig. 1 diagrammatically shows a receiving system embodying the invention,

Figs. 2, 3, and 4 show the various operating characteristics of the system in Fig. 1,

Fig. 5 shows a circuit diagram of a modified form of the invention,

Fig. 6 graphically shows the operation of the system shown in Fig. 5,

Fig. 7 shows another modication of the in- Vention,

Fig. 8 illustrates still another embodiment of the invention.

Referring now to the accompanying drawings, wherein like reference characters in the difierent figures represent similar circuit elements, there is shown in Fig. 1 a receiving system re- Those skilled in the art will fully understand that the present invention is to be applied to modern broadcast lreceivers of the superheterodyne or tuned radio frequency types. In Fig. 1 the essential networks include the controlled signal amplifier which vmay comprise a tube i of the screen grid type. The input electrodes of the tube are coupled to a resonant input circuit 2, the input circuit including the tuning condenser 3 for tuning the input circuit 2 to a desired signal frequency.

The input circuit 2 is coupled, as at Mi, to any desired source of signal energy. The anode of the tube l is coupled, as at M2, to the tuned input circuit l of the following screen grid tube 5. The anode circuit of tube 5 includes a radio frequency filter network 6 for preventing the ow of radio frequency currents to the following audio frequency network, the latter not being shown.

It is to be understood that the tube l functions as a radio frequency amplifier, and the tube 5 functions as a signal rectifier, or detector. When the present invention is to be applied to a superheterodyne type of receiver, then the amplier l may be of the intermediate frequency type, and the detector 5 will be the second detector. One, or more, stages of audio frequency amplication may follow the detector 5. The signal source coupled to input circuit 2 would, in that type of receiver, comprise the usual signal amplier iollowed by a rst detector network. In the superheterodyne receiver, of course, the tuning condensers in the input circuits of tubes l and 5 would be adjusted to maintain the circuits 2 and 4 xedly tuned to the operating intermediate frequency.

If the receiver is of the tuned radio frequency type, then the tuning condensers of the input circuits 2 and 4 are to be considered as continuously variable, and any desired type of mechanical uni-control may be utilized to vary the rotors of the condensers. In such a case the signal source preceding circuit 2 will usually include one, or more, additional stages of tuned radio frequency amplification. Regardless ci which type of receiver is employed, it is to be understood that the automatic control circuit to be described can be used to control the amplification of the various tubes preceding the detector 5. In other words, in the superheterodyne type of receiver the AVC (hereinafter used to designate the automatic volume control network) network to be described can be used to control the amplification of the radio frequency amplifier, the first detector and the intermediate frequency amplifier.

The electrodes of the controlled tubes and the detector tube are energized from the usual bleeder resistor of the power supply network, and there has been shown in Fig. l the rectifier iilter portion of the power supply network, as well as the supply resistor R3. One side of the resistor is grounded, while the other side is the +B side of the supply network. Those skilled in the art are fully aware of the points on the supply resistor R3 to which the diiierent electrodes of the tubes of the receiving system are to be connected for proper energization. The cathode of controlled amplifier i is shown connected by an adjustable lead to an intermediate point on resistor R3. Bias for the signal control grid of tube i is secured by grounding the low alternating voltage side of input circuit 2, and the signal grid of the ampliiier I will then be at a negative voltage with respect to the cathode of tube i; this voltage is equal to the voltage drop across the portion of resistor R3 between ground' and the adjustable tap of lead Tl. This method of providing a bias for the controlled amplier is utilized as it provides a bias which is substantially unaffected by the amount of space current flowing in the amplier. In order to economize on rectified current the cathode of tube i may be tapped onto resistor R2, as this resistor is also at a positive potential. Again, the cathode of tube l may be tapped onto resistor Ri, but for the sake of simplicity these three resistors are shown separately so that each one has only one function.

Grid bias for the detector tube 5 is provided by the flow of detector space current through resistor Ri in the customary fashion. How-ever, it will be noted that resistor Ri is arranged in series with the magnetic field winding wi. It is pointed out that the resistor Ri may be wholly, or in part, the resistance of the field winding wi. The condenser Ci shunts the field winding and resistor Ri in the usual fashion to by-pass audio ire-- quency currents. If desired, this condenser can be made large enough to by-pass variations in voltage space current clue to the more rapid changes in modulation intensity. The function of winding wi is to provide a magnetic eld acting upon magnetron tube M.

The magnetron tube is a controlled space discharge device and its construction is well known to those skilled in the art. The electron emission element, or cathode, of the magnetron is connected to ground through a resistor R2, the screen grid electrode of the controlled amplifier l being connected by lead iii to a point on resistor R2 adjacent the cathode of the magnetron tube. The tap ill of lead l@ enables the lead to be adjusted to any desired point on resistor Rz. A by-pass condenser C2 is connected between lead l@ and the grounded side of resistor R2. The anode of magnetron tube M is connected to a suitable positive voltage point on the bleeder resistor R3.

The magnetron tube is, additionally, provided with a second eld winding wi, one end of which is connected to an intermediate point on the resistor Ri, one side oi this resistor being grounded. The other side of resistor Ri is connected to a suitable point of positive potential on the voltage supply resistor. The opposite end of the winding wi is connected by an adjustable lead Il to any desired point on resistor Ri.

2 is a graphical representation of the dependency ci the magnetron current im upon the total field impressed on magnetron M. It will bo seen that the current is constant until the field reaches a certain critical value, and that a further increase of field intensity rapidly shuts oiI the magnetron current approximately completely.

The solid curve in Fig. 3 shows the relationship between the magnetron current and the direct current component or the detector current id. The direct current component of the detector space current flows through the field winding wi, and the solid line curve oi 3 shows how the magnetron current im depends upon the current id. It will be observed that this solid line curve is of exactly the same shape as that in 2, because the field on the magnetron varies directly with the current through 'the eld winding wi.

The field winding wi oi the magnetron tube, also, acts upon the magnetron, and its terminals are adjustably tapped upon resistance R4, the latter being energized by direct current, so that a iiXed field of reversible sign and adjustable magnitude can be impressed upon the magnetron by winding wi. I'Ience, the total field upon the magnetron is the sum of the s produced by the two windings wi and wi. In F 3 the dotted line curve d shows the characteristic when the iiXed eld due to winding wi is made to aid the field due to windingtm, while the dotted curve b shows the :characteristic when the iields ofthe two windings are in opposition. f f' It will, thus, be seen that .by suitable adjust ment of the current through winding wl, the mag netron current can be caused to'cut off aty any current, or la part of this voltage, is applied to the screen electrode of 1the'radio frequency amplifier tube l. When the magnetron current is shut ofi as a result of the detector currentincreasing to the critical value, the voltage on the screen of the controlled ampliiier vanishes. Since the detector current is increasesas the input to the detector increases, an over-all characteristic as shown in Fig. 4 results.

The curves of Fig. ll are obtained by plotting amplication of the controlled ampliiier against signal input to the detector. Fig. 4 shows how the radio frequency amplification remains constant up to a certain value oi detector signal input, and then drops rapidly to Zero for a slight increase in detector signal input. The dotted line curves a and b correspondto those of Fig. 3v; it is evident that the adjustment of the fixed field produced by winding un provides a method of adjusting the detector input level which is to be maintained constant by the automatic volume control action.

In Fig. 5 there is shown a mcddiication of the circuit arrangement of Fig. l. In this arrangement the rectifier tube of the power supplyV system is shown, although the alternating current source is omitted. The rectiiier filter F includes in its negative side the speaker iield, and the bleeder resistor R3 is again shown connected across the output of the filter F. The positive side of resistor R3 is connected in the well known fashion to the vario-us anodes of the tubes. The anode of the magnetron tube M is connected by lead l2 to a suitable point on resistor Ra, while the electron emission element, or cathode, of the magnetron is connected by lead l@ to the screen grid electrode of the controlled amplifier i. The cathode of the controlled amplifier is connected to ground through the usual grid biasing network I3, and a radio frequency by-pass condenser is connected between the screen` grid and cathode of the controlled amplifier.

The intensity of the xed eld winding wir is controlled, in this form of a circuit, by connecting the terminals oi the winding to resistors R4 and R5. The resistor R4 is arranged in the positive side of the rectiiier filter F, and the resistor Rt is connected in series with resistor R4 and with the winding un. The reference letter A represents a device for uni-controlledly varying inversely the resistances of resistors R4 and R5. Adjustment of element A results in an adjustment of the current flow through winding un without changing the filter terminal voltage. Again, as in the case of Fig. l, the resistance R1 may be the resistance of winding w1. The portion of resistor R3 between ground and the tap point of lead i2 may be omitted, and the upper portion of this resistor may be shorted if found: satisfactory. Variation of the control A adjusts the level at which the vdetector carrier input is maintained. Fig. 6 graphically shows the relation between screen current flow and detector space current in the circuit shown in Fig. 5. The dotted linecurve :c shows the relation between the two currents for a large current iiow through the iield winding wi; solid line curve y shows the characteristic when a medium current flows through this field winding; and dotted linecurve z shows the same characteristic for a small current flow through the" field winding wir, the fields being assumed to be additive in each case. Y

It is not necessary to the present invention to regulate amplification ofA the radio frequency ampliers by varying the screen grid voltage.

For example, and as shown in Figs. 7 and 8, the

signal control grid voltage can be adjusted in response to variations in the detector space current flow. In Fig. 7 the controlled amplifier l has its signal grid 2d connectedto the electron emit-` ter of magnetron M by the AVC lead` 2l; the usual ripple suppression resistor 22 and condenser 23 are included in the AVC connection. Resistor R5 is connected between the emitter of magnetron lVl and the ungrounded side of reactor 24, and the latter may function as the speaker field.

The detector space current flows through magnetron iield winding w1; the adjustment device A-Ri-Rf, again is used to determine the level of the detector signal input which is to be main-I tained. The current iiow through winding wi is adjusted to permit iiow of current through resistor R6 in the presence of weak signals. This results in placing the signal grids of the controlled ampliers-at substantially ground potential when received signals are weak. As the intensity of the detector space current increases the iield of winding w1 adds to that of the field of winding wi, and stops the current flow through resistor Re. This, of course, results in the impression of ,a large negative bias on the signal grids since ground, and the condenser 23 is connected across resistor Re. The AVC lead 2i is connected between the signal grid of the controlled amplier and the cold electrode side of resistor Rs. The magnetron emitter is connected tothe ungrounded side of reactor 2li. The adjustment device A, maintaining a constant impedance to flow oi current through filter F, can be set so asl to adjust the current through winding wi to cut-off the current through the magnetron lVi until a desired amount of detector space current iows through winding w1. The latter is connected so as to reduce the total field on the magnetron. If' the detected current exceeds this predetermined value, current flows through the magnetron and resistor Re.

In this way additional bias is applied to the -controlled amplifier grids when signal input to the detector increases. The detector input is held at a substantially fixed value determined by the adjustment of device A. In brief, then, in Fig. 8 thema'gnetron current is cut off for weak signals, thus xing the controlled amplifier grids at ground potential; the fields of windings w1 and 'un buck each other whereby when the detector current exceeds a predetermined value,

magnetron current liows and causes the signal grids to become negative With respect to their cathodes.

While I have indicated and described several systems for carrying my invention into eiect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modifications may be made without departing from the scope, as set forth in the appended claim. Y

What I claim ist- A radio receiver having a controlled signal transmission tube and a detector, a common direct current voltage Supply source, including a bleeder resistor, for the transmission tube and detector, a magnetron gain control tube, a direct current connection from the anode of said magnetron tube to a point of proper positive potential on said bleeder resistor, a eld winding for said magnetron tube disposed in the space current path of said detector, a second field winding for said magnetron tube, a pair of resistors connected in series with said second iield winding, one of said resistors being disposed in series with said bleeder resistor, a gain control connection between a gain control electrode of said transmission tube and said magnetron tube, and means for simultaneously varying the magnitudes of said two series resistors in inverse relation whereby the direct current flow through the second eld winding may be adjusted without any substantial effect on the voltage across said bleeder resistor. i

WALTER VAN B. ROBERTS. 

